Non-volatile split gate eprom memory cell and self-aligned field insulation process for obtaining the above cell

ABSTRACT

The cell comprises a substrate with diffusions of source and drain separated by a channel area a floating gate superimposed over a first part of said channel area and a control gate formed by a first and a second polysilicon strip, respectively, a cell gate oxide between said floating gate and said first part of the channel area, a transistor gate oxide between said control gate and a second part of the channel area, an interpoly oxide between said floating gate and said control gate and a layer of dielectric filler. By means of a process which provides for self-aligned etchings of layers of polysilicon and of oxides there is obtained a floating gate and a control gate self-aligned with one another and with the diffusions of source and drain, as well as with the first oxide.

DESCRIPTION

The present invention relates to a non-volatile split gate EPROM memorycell and self-aligned field insulation process for obtaining the abovecell.

The use is known of non-volatile split gate EPROM memory cells. In suchcells the floating gate does not extend along the entire channel of thecell, but only covers a part of it to form the actual memory cell. Thesecond part of the channel is governed directly by the control gatewhich thus goes to form a small transistor in series with the cellitself.

A first advantage deriving from the use of the split gate memory cellconsists in the fact that with such a structure the problem ishighlighted of the partial raising of the floating gate voltage (withconsequent current in the channel) due to the capacitive couplingbetween floating gate and drain (a problem known as <<drain turn-on>>),when the drain is biased at a high voltage and the control gate isgrounded. In fact in a split gate cell, even in the case when thecapacitive coupling between the floating gate and the drain is large,the opening of the channel is completely prevented by the presence ofthe transistor in series with the cell itself.

A second advantage deriving from the use of the above memory cellconsists in the fact that, thanks to its asymmetry, it can be programmedstarting from only one of the two diffusions (the one known as thedrain, that is, that facing the floating gate). If the cell is read fromthe source side (that is, by raising above ground the diffusion fromwhich writing is impossible), it is then possible to abolish completelythe so-called <<soft-writing>> problem, that is, of the slow andundesired programming of the cell during its reading step.

As a result of this advantage the voltage at which the bit-linediffusion is polarised during the reading step of the cell itself can beraised with respect to the typical value used (about 1 volt), thusobtaining both an increase in the cell's reading current and a drop inthe capacity of the bit-line itself.

Several types of split gate EPROM memory cells are known in theliterature.

A first example of a cell of this type is described in the U.S. Pat. No.4,328,565 and consists of a non self-aligned structure, in which thesource and drain diffusions are implanted before the definition of thesuperimposed floating and control gates.

It follows that, for the drain diffusion to be coupled to the floatinggate (that is, sufficiently superimposed over it) independently of themis-alignments between the different masks, it is necessary to maintaina large superimposition between the floating gate itself and the drain,from which there derives a large and highly variable capacitive couplingbetween the latter.

During the cell's programming step this is reflected in a widevariability of the value of the voltage transferred to the floating gateand thus in a threshold jump, after the writing step, whose value ispredictable with difficulty and a function of the process misalignments.

A second problem related to this cell is connected with the fact ofexecuting the diffusion implant before growing the subsequent oxidelayers of gate and interpoles. The diffusions are subjected in this wayto all the heat treatments connected with these oxidation steps and theyare thus deper and more diffuse. This is the opposite of the currenttendency towards new generations of processes characterised by anincreasingly higher density.

A last problem related to this type of cell is connected with the factthat the control gate passes over the gate and drain diffusions, itbeing insulated from them only by a thin layer of thermal oxide. Itfollows that the word-line capacity is raised with a correspondingreduction in the speed of the device.

A second example of cell of this type is described in U.S. Pat. No.4,639,893.

In this cell the drain diffusion is self-aligned with the floating gate;the source and drain implant is executed after defining the floatinggate itself and, on the drain, it is self-aligned with it. Thecapacitive coupling of the floating gate with the drain thus becomesindependent from the process misalignments.

The distance between source and drain, on the other hand, is subject tothe misalignment between the mask which defines the floating gate andthat used for the source and drain implant.

As in the cell of the first example, the control gate passes completelyover the source and drain diffusion and it is insulated from it only bya thin oxide thermally grown simultaneously with the gate oxide and withthe interpoly one.

The greatest problem of this type of structure is precisely in thisoxidation step; to lower the word-line's capacity (and thus to raise theaccess speed of the device) it is in fact necessary for the thin oxideto be as thick as possible, but this is not reconcilable with a goodquality of the other two oxides. To obtain an appreciable differencebetween the thickness of these oxides it is in actual fact necessary toexecute oxidation at low temperatures, but the oxides so obtained are ofpoor quality.

In view of this state of the art, the main object of the presentinvention is that of accomplishing a split gate EPROM memory cell thatdoes not have the above drawbacks.

A further object of the present invention is that of accomplishing aprocess for obtaining the above cell.

According to the invention such object is attained with a non-volatilesplit gate EPROM memory cell, comprising a substrate with diffusions ofsource and drain separated by a channel area, a floating gatesuperimposed over a first part of said channel area and a control gateformed by a first and a second polysilicon strip, respectively, a cellgate oxide between said floating gate and said first part of the channelarea, a transistor gate oxide between said control gate and a secondpart of the channel area, an interpoly oxide between said floating gateand said control gate and a layer of dielectric filler, characterised inthat said floating gate is aligned with the drain diffusion and saidcontrol gate is aligned with said floating gate and with the diffusionsof source and drain.

Again according to the invention such further object is attained througha process, characterised in that it comprises the following steps:definition of parallel strips of insulation oxide of field and activearea and execution of a cell channel implant on a semiconductorsubstrate; growth of cell gate oxide, deposition and doping of a firstlayer of polysilicon and definition of both in first stripsperpendicular to those of the field insulation; growth of transistorgate oxide of the transistor and of interpoly oxide, deposition anddoping of a second polysilicon layer; definition of said secondpolysilicon layer in second strips parallel and partially superimposedover those of the first polysilicon layer; self-aligned etching of themultistrate comprising the second polysilicon layer, the transistor gateoxide, the interpoly oxide, the first polysilicon layer and the cellgate oxide up to the field insulation oxide and subsequent etching ofsaid field oxide with uncovering of substrate strips; execution ofimplants of source and drain in said uncovered strips of the substrateand subsequent lateral sealing oxidation of the multistrate; fillingwith thick dielectric of the empty spaces between one cell and the next.

In this way there is obtained an EPROM memory cell in which the draindiffusion is self-aligned with the floating gate and with the controlgate, in turn self-aligned with one another, and the source diffusion isself-aligned with the control gate, and wherein the distance between thesource and drain diffusions is determined by the width of the maskdefining the control gate and is thus independent of the misalignmentbetween the masks. In addition the source and drain junctions are notvery diffused as they are not subjected to any of the oxidation heattreatments for the formation of the gate oxide and of the interpolyoxide.

Moreover, in this cell, the presence of a thick dielectric allows theword-line, constituted by a third series of polysilicon or silicidestrips perpendicular to those defining the control and floating gates,to be arranged very high up, thus avoiding the problem of the tripleoxidation and of the high coupling capacity between the gates and thesource and drain diffusions.

Finally the self-alignment of the field insulation with the control gateallows the cells to be packed into a matrix in a periodic structurehaving a so-called <<tablecloth>> pattern in which it is not necessaryto have a drain contact for each pair of cells.

The features of the present invention shall be made more evident by thefollowing detailed description of an embodiment illustrated as anon-limiting example in the enclosed drawings, wherein:

FIG. 1 shows a vertical sectional view of a pair of cells according tothe invention;

FIGS. 2-4 show a top plan view and a sectional view, taken along thelines III--III and IV--IV of FIG. 2, respectively, of the initial stepof the process for manufacturing the cells of FIG. 1;

FIGS. 5-7, 8-10, 11-13, 14-16, 17-19, 20-22, 23-25, 26-29 and 30-33,with views similar to those of FIGS. 2-4, show in turn subsequent stepsof the above process.

With reference to FIG. 1, each cell 30 comprises a semiconductorsubstrate 20 in which there are obtained diffusions of source 11 anddrain 12 essentially of the type n+, between which there is a channelformed by adjacent portions 7, 8, preferably doped in a different way.Over the portions 8 and 7 there are superimposed a layer of cell gateoxide 3 and a transistor oxide layer 9', respectively. Over oxide 3there is in turn superimposed a first polysilicon strip (perpendicularto the plane of the drawing) constituting a floating gate 4, over whichthere is an interpoly oxide 9 similar to the transistor oxide 9'. Overoxides 9 and 9' there is arranged a second polysilicon stripconstituting a control gate 10. As shown, the floating gate 4 is alignedwith the drain diffusion 12 and the control gate 10 is aligned with oneedge of the floating gate 4 and with the source and drain diffusions 11and 12.

A dielectric 14 is used for sealing the spaces present between one celland the other and lastly a further polysilicon or silicide strip 15,constituting a connection word-line between cells, extendsperpendicularly to the gate strips 4 and 10.

With reference to FIGS. 2-28, the process according to the invention forthe accomplishment of the pair of cells 30 of FIG. 1 comprises a firststep in which on the silicon substrate 20 there are defined parallelinsulations strips of field 1 and of active area 2 (FIGS. 2-4). Duringsuch step there is also executed the channel implant of the cell(portions 8 and 7 of FIG. 1), necessary for defining the thresholdvoltage.

With reference to FIGS. 5-7, there is then executed a growth of cellgate oxide 3 and the subsequent deposition of a layer of polysilicon 4.The layer of polysilicon 4 is made conductive (doped) using conventionaltechniques.

With reference to FIGS. 8-10, the double layer of cell gate oxide 3 andpolysilicon 4 is defined in strips perpendicular to the field oxidestrips 1. After etching the polysilicon layer 4 and of the cell gateoxide 3, there may be executed, if necessary, an implant of transistorchannel (portion 7 of FIG. 1), whose object is to define the thresholdof the transistor in series with the cell; in this way, as shown in FIG.1, the channel doping in the portion of channel 7 directly controlled bythe control gate 10 is differentiated with respect to that in theportion of channel 8 under the floating gate 4.

With reference to FIGS. 11-13, the growth is executed of transistor gateoxide 9' and of interpoly oxide 9, over which there is deposited andmade conductive a second layer of polysilicon 10 which shall constitutethe control gate.

With reference to FIGS. 14-17, there is first accomplished thedefinition of a second layer of polysilicon 10 in strips that areparallel and partially superimposed over those of the first layer 4 andthen the self-aligned etching of the multilayer constituted by the firstand second layer of polysilicon 4, 10, by the interpoly oxide 9 and bythe gate oxides 9' and 3 according to strips parallel to one another andperpendicular to the strips related to the field oxide 1 and to theactive regions 2. With subsequent etching of the field oxide 1 there areuncovered side-by-side regions of source 11 and drain 12, constitutingthe bit-lines, aligned with the control gate 10 and with the floatinggate 14, respectively. During the etching of the first layer ofpolysilicon 4 the cell's asymmetry (FIG. 15) determines the creation ofsmall excavations 30 in the silicon substrate 20 in the source areas 11.

With reference to FIGS. 18-21, there is executed an n⁺ implant in theregions of source 11 and of drain 12, after which a lateral sealingtakes place with oxide 13 of the multistrate 3, 4, 9, 10.

With reference to FIGS. 22-25, the planarisation is executed, that is,the filling with dielectric 14 of the spaces between the cells 30.Typically, planarisation is executed by depositing a layer of deposedoxide and subsequently a layer of <<spin-on-glass>> (or resit) whoseobject is to make the surface planar.

Planarisation is completed by partially removing the dielectric 14 andthe covering oxide 13 over the second layer of polysilicon 10, so thatthe polysilicon surfaces 10 are not covered with oxide and may thus becontacted.

With reference to FIGS. 26-29, there is executed the deposition of aconductive layer 15, typically polysilicon or silicide, which shallconstitute the word-lines.

With reference to FIGS. 30-33, the self-aligned etching is executed ofthe multistrate 3, 4, 9, 10, 15 up to the field oxide 1, according tostrips which are parallel to, but wider than, the active area strips 2,so as to obtain cells with lateral fins superimposed over the fieldoxide 1 (FIG. 32). In this way the fins of one cell are separated fromthose of the contiguous cell.

The steps of the standard process which follow, in particular thesealing reoxidation, the deposition of intermediate dielectric, theopening of the contacts and the metallisation, are not described here.

I claim:
 1. A non-volatile split gate EPROM memory cell, comprising:asubstrate with diffusions of source and drain separated by a channelarea; a floating gate superimposed over a first part of said channelarea; a control gate formed by first and a second polysilicon strips,respectively; a cell gate oxide between said floating gate and saidfirst part of the channel area; an interpoly oxide between said floatinggate and said control gate; and a layer of dielectric filler located toseal spaces present between the cell and an adjacent cell; wherein saidfloating gate is aligned with the drain diffusion and said control gateis self-aligned with said floating gate and with the diffusions ofsource and drain.
 2. The cell of claim 1, further comprising a fieldoxide region self-aligned with the floating and control gates, the widthof said field oxide region being determined by the second polysiliconstrip.
 3. The cell of claim 1, further comprising a field oxide regionand wherein the floating gate has lateral fins superimposed over thefield oxide region so as to increase capacitive coupling with thecontrol gate.
 4. The cell of claim 1, further comprising a strip ofconductor material superimposed over and in contact with said secondpolysilicon strip for making an electrical connection with adjacentcells, said strip of conductor material being separated from saiddiffusions of source and drain by said dielectric layer so as to reducecapacitive coupling with said diffusions.